1. Field of the Invention
The present invention relates to a MOS technology power device, particularly a power MOSFET or an Insulated Gate Bipolar Transistor (IGBT).
2. Discussion of the Related Art
Among the most important electric parameters which characterize power MOS devices are the output resistance (R.sub.DSon), the input capacitance (gate-source capacitance or gate-emitter capacitance) and the feedback capacitance (gate-drain capacitance or gate-collector capacitance).
The output resistance is important during steady-state conditions because the output resistance determines the power dissipation of the power device, and should therefore be as small as possible.
The input and feedback capacitances are instead important during switching because the capacitances determine the turn-on and turn-off time of the power device. Longer turn-on and turn-off times cause a higher power dissipation during switching and limit the operating frequency of the power device.
In order to reduce the input and feedback capacitances, it is necessary to increase the gate oxide thickness and/or to reduce the extension of the area of superposition between the gate electrode, the channel region and the drain layer.
Unfortunately, an increase in gate oxide thickness or reduction in the area of superposition negatively affects the steady-state characteristics of the power device, such as the output resistance. In fact, an increase of the gate oxide thickness would cause an increase of the channel region and accumulation components of the output resistance. A reduction of the distance between the body regions of the elementary cells (or stripes) of the power device in an attempt to reduce the superposition area between the gate electrode and the drain layer would cause an increase of the so-called JFET component of the output resistance.
Conventionally, the distance between the elementary cells or stripes of the power device depends on the lateral diffusion of the body regions and on the requirement that the JFET component of the output resistance is to be kept low. For example, in power devices for low-voltage applications the distance between the elementary cells or stripes of the power device cannot be lower than 5-6 .mu.m.
In the co-pending European Patent Application No. 95830453.7 filed on Oct. 30, 1995 in the name of the same Applicant, a high-density MOS technology power device is described comprising rectilinear elongated apertures in the insulated gate layer under which rectilinear elongated P type body stripes are formed in an N+ drain layer. The body stripes extend laterally under the insulated gate layer symmetrically with respect to the symmetry axes of the elongated apertures in the insulated gate layer. Inside each body stripe, a plurality of N+ source regions are formed which are disposed in longitudinal succession on opposite sides with respect to a symmetry axis of the body stripe. In this way, it is possible to reduce the distance between adjacent elongated apertures in the insulated gate layer without increasing the JFET component of the output resistance, because the portions of the drain layer positioned between adjacent body stripes experience a current flux from only one side. Device simulations have in fact proved that the output resistance depends on the current density in the portions of the drain layer comprised between adjacent body stripes: an asymmetric current flux allows therefore a reduction of the distance between the body stripes without increasing the output resistance.
However, even in this power device structure the distance between adjacent elongated apertures in the insulated gate layer cannot be reduced significantly because of the symmetrical lateral diffusion of the body stripes under the insulated gate layer.